Fix PR26622 - Make CheckLibcxxAtomic.cmake use the libc++ headers.
Fix PR26622 - Make CheckLibcxxAtomic.cmake use the libc++ headers.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@261383 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx